In a cellular mobile communication system, the coverage area is divided into a number of cell sites each having antennas and associated equipment for transmitting and receiving signals. A base station controls the cells through an interconnecting network of high speed data and voice links between the cells and the base station. As traffic increases, a large cell may be split into smaller cells, thereby requiring additional high speed links to complete the network.
As indicated by Lee in "Mobile Cellular Telecommunications Systems," McGraw-Hill Book Company, pp. 337-354 (1989), either T1 lines, 800-MHz radio links, or microwave radio links may be used between the base station and cells.
In the cellular mobile phone system disclosed by Ehrlich et al. in "Advanced Mobile Phone Service: Cell Site Hardware," Bell System Technical Journal, Vol. 58, No. 1, pp. 153-199 (1979), a data bank is installed at each end of the T1 carrier. The data bank converts 20 voice channels along with system control data into a 1.54-Mbps data stream for transmission by a T1 cable. At the receiving end, the voice channels are demultiplexed and then sent to the radio frame for final radiation by an antenna.
In a 6-sector site supporting 90 radio channels (15 radio channels per sector), for example, four and one-half T1 cables are required for each direction. Disadvantageously, the use of T1 lines creates the possibility of service interruptions to the cellular mobile service provider since T1 lines are usually leased from telephone companies and are thereby subjected to rearrangements by the telephone company.
A microwave link operates similarly to the T1 cables, except that the T1 cables are replaced by a modulated microwave carrier and free space is used as the transmission medium. Although microwave links have the capacity to transmit as many as 900 channels, the links are subjected to the constraints of frequency coordination and outage due to multipath.
The frequency bands assigned to cellular mobile phone service are 824 MHz to 849 MHz for mobiles and 869 MHz to 894 MHz for land. The 25-MHz bandwidth is divided into two equal sets and awarded to two competing service providers in each area. Each 12.5 MHz bandwidth is divided into 416 channels with a bandwidth of 30 KHz. The total number of channels is divided into 21 channel-sets for frequency reuse. In each set, adjacent channels are spaced 20 channel-bandwidth apart to reduce adjacent channel interference. For instance, in a six-sector cell site, six different channel-sets are used. Though all channel frequencies are distinct, they nevertheless occupy the same 12.5 MHz bandwidth.